In inkjet printing methods, an image is printed by ejecting ink droplets onto selective locations of a printing medium such as, for example, a sheet of paper. Inkjet printing methods are widely used because high resolution and high quality images can be obtained using a relatively simple approach. However, the demands for a technique for obtaining higher optical concentration and higher image quality are continually increasing. Several conventional techniques for higher optical concentration and higher image quality are described below.
One conventional technique uses black ink that agglomerates when reacting with a salt and a color ink. According to this method, when an image is printed, an agglomeration phenomena occurs between the salt and the black and color inks, which provides a high quality image while preventing color bleeding. That is, a good image can be obtained by using two types of inks that have different ionic characteristics, and a salt to induce an agglomeration phenomena between the two inks. The salt used in this case may be a polyvalent metal salt.
Another conventional technique uses a colorless reaction solution instead of a salt in an ink. According to this method, ink consists of a colorant having an opposite charge to that of a salt used in the colorless reaction solution. Ion pairing is induced on the surface of paper during printing so as to cause an agglomeration phenomena. The colorless reaction solution used in this case may include a polyvalent salt and/or a cationic polymer.
As described above with regard to the conventional techniques, due to the fast reaction between ink and a reaction solution having different ionic groups, many ink residues remain on the surface of paper which leads to the formation of high resolution and high quality images. However, since ink is mostly in a liquid state due to the inherent characteristics of inkjet printing, the printing paper curls due to the moisture present in the ink after image printing. That is, a curling phenomena occurs. A reaction solution including a polyvalent metal salt and a cationic polymer alone may not prevent such paper curling.
Accordingly, there is a need in the art to develop an inkjet printing technique for producing high optical concentration and high quality images with reduced curling of the printing medium.